1. Field of the Invention
The present invention relates to a method for manufacturing 2,6-dimetylnaphthalene which can be used effectively as a feedstock for a monomer such as 2,6-naphthalene dicarboxylic acid which is used for forming polyesters such as polyethylene naphthalate.
2. Description of the Related Art
In order to obtain the superior features of polyethylene naphthalate which is used for manufacturing fibers, films, and the like, it is necessary that 2,6-naphthalene dicarboxylic acid, which is a monomer for forming polyethylene naphthalate, has a high purity, and accordingly, it is also desirable that 2,6-dimethylnaphtahlene which is a precursor therefor has a high purity. Dimethylnaphthalene (hereinafter referred to as xe2x80x9cDMNxe2x80x9d) has 10 isomers, and a highly pure 2,6-dimethylnaphthalene (preferably having a purity of 99% or more), which is mixed with substantially none of the other 9 isomers, is preferably used for forming 2,6-naphthalene dicarboxylic acid.
As a method for manufacturing the 2,6-DMN described above, there may be mentioned a method of separating 2,6-DMN from a mixture obtained by isomerizing 1,5-DMN which is formed by a reaction between orthoxylene and butadiene; a method of separating 2,6-DMN from a mixture obtained by disproportionating methyl naphthalene or isomerizing dimethylnaphthalenes and a method of separating 2,6-DMN from a tar or an oil fraction. However, the fractions and the mixtures described above are each a mixture containing many types of DMN isomers in addition to 2,6-DMN, and hence, the 2,6-DMN must be separated from the mixture described above. However, since the boiling points of these DMN isomers are very close to each other, it has been difficult to separate a highly pure 2,6-DMN therefrom by distillation which is commonly used for separation/purification of organic compounds.
Accordingly, as a method for separating 2,6-DMN, a crystallization method or an adsorption method has been proposed, and in addition to these methods mentioned above, a method comprising steps of forming a complex by using a certain organic compound, separating the complex, and decomposing the separated complex, and combinations of the methods described above have also been proposed. A cooling crystallization method is a method exploiting the property of 2,6-DMN having a highest melting point among the 10 types of DMN isomers, and since the cooling crystallization method is simple compared to the methods described above, this method can be used suitably as an industrial separation method. However, since it has been difficult to obtain a 2,6-DMN having a purity of 99% or more only by the cooling crystallization method, a process such as treatment using a solvent is generally used together therewith. For example, in Japanese Unexamined Patent Application Publication Nos. 48-5767 and 48-22449, and Japanese Examined Patent Application Publication No. 50-22553, a method has been disclosed in which after a mixture of DMN isomers is crystallized by cooling, solid-liquid separation was performed by suction filtration, and the obtained solid component is dissolved in a solvent and is then crystallized by cooling. However, in the method disclosed in the publications described above, the mixture of DMN isomers used as a feedstock primarily contains specific DMN isomers among the 10 types of isomers, such as 2,6-DMN, 1,6-DMN, and 1,5-DMN, which are easily isomerized to each other and are easily separated, and the content of 2,7-DMN which is difficult to separate from 2,6-DMN is limited to less than 5 mole percent (approximately equivalent to 5 wt) or less. A mixture of DMN isomers obtained in a typical manufacturing process generally contains 5 wt % or more of 2,7-DMN, and hence, when the mixture of DMN isomers containing 5 wt % or more of 2,7-DMN is used as a feedstock in accordance with the related art disclosed in the publications described above, it has been difficult to obtain a highly pure 2,6-DMN.
In view of the problems described above, the present invention was made, and an object of the present invention is to provide a method for manufacturing a 2,6-DMN having a purity of 99% or more even when a mixture of DMN isomers containing 5 wt % or more of 2,7-DMN is used as a feed stock.
To this end, a method for manufacturing 2,6-dimethylnaphthalene according to the present invention comprises a step of performing at least one crystallization and at least one solid-liquid separation of a liquid primarily containing dimethylnaphthalene isomers used as a feedstock so that the liquid is separated into a cake containing dimethylnaphthalene isomers and a mother liquor, and so that the content of 2,6-dimethylnaphthalene is increased in the cake; and a step of performing separation/purification of the cake. In the method described above, the crystallization and the solid-liquid separation are performed under the condition in which the ratio of the content of 2,6-dimethylnaphthalene in the mother liquor to that of 2,7-dimethylnaphthalene therein (hereinafter referred to as xe2x80x9cratio 2,6-DMN/2,7-DMNxe2x80x9d in some cases) is not less than 1 so that the content of 2,6-dimethylnaphthalene in the cake is 60% or more and that the content of 2,7-dimethylnaphthalene therein is 6.5% or less, whereby a 2,6-dimethylnaphthalene having a purity of 99% or more is obtained by performing the separation/purification of the cake.
According to the method of the present invention described above, a highly pure 2,6-DMN can be manufactured even when the mixture containing dimethylnaphthalene isomers contains 5 wt % or more of 2,7-dimethylnaphthalene is used as a feedstock.
In addition, in the step of performing the crystallization and the solid-liquid separation described above, it is preferable that after cooling crystallization is performed, press filtration be performed as the solid-liquid separation.
In the present invention, xe2x80x9c%xe2x80x9d means xe2x80x9cwt %xe2x80x9d.